This book provides a fundamental introduction to Aquinas's theology of the One Creator God. Aimed at making that thought accessible to contemporary audiences, it gives a basic explanation of his theology while showing its compatibility with contemporary science and its relevance to current theological issues. Opening with a brief account of Aquinas’s life, it then describes the purpose and nature of the Summa Theologica and gives a short review of current varieties of Thomism. Without neglecting other works, it then focuses primarily on the discussion of the One God in the first part of the Summa Theologica. God's transcendence and immanence is a recurrent theme in that discussion. Evidence of God's immanent causality in the natural world grounds Aquinas's five arguments for the existence of God (the Five Ways) which then open onto God's transcendence. The subsequent discussion of the divine attributes builds on the modes of God's causality established in the Five Ways. It also shows the need for a language of analogy to preserve God's transcendence and prevent us from reducing God to the level of creatures, even as qualities such as "goodness" and "love," which we first know from creatures, are applied to God. The discussion of God's providence and governance establishes that the transcendent Creator God is most intimately present in creation. God acts in all creatures in a way that does not diminish their proper causality, but is rather its source. As there is no contradiction between God's transcendence and immanence, so there is no competition between the primary causality of God and the secondary causality of creatures. Empirical science, which is limited by its method to the secondary causality of creatures, is shown to be compatible with the broader discipline of theology which also embraces the primary causality of the Creator.